The Tschirnhausen Transformation
by VioletBaudelaire07
Summary: Don likes math talk... Oneshot. Don/Spencer. Prompted by 'Absolut' by hctiB notsoB. Warning: Eppescest ist mentioned.


**The Tschirnhausen T****ransformation**

**Just so you know: I'm not into incest stories, not at all actually. However, I saw this story as something of a challenge for myself, to see whether I can do this convincingly or not. I find it rather weird if incest-stories just end with: 'well, we're relatives but who cares, I love you'. I think for those involved it's a really difficult situation and I tried to put that into words here.  
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**Prompted by** – _Absolut_ by hctiB-notsoB (and my current obsession with Rob Morrow. Alpha males are definitely my thing... hrrr!)

**Warnings** – M for language, graphic M/M slash, and mentioning of incest.

**Pairing** – Don/Spencer

* * *

><p>Don woke with a gasp, breathing hard. The first thing he noticed was that he was having the worst headache he probably ever had. Then he remembered his dream – <em>No, nightmare!<em>, he corrected himself, _I must remember to call them nightmares_ – he just had had. _Not good_. He felt that his knees were still a bit weak from the overpowering sexual release he had just experienced. At the same time the well known nauseating feeling in his stomach was back. He shouldn't be having these dreams, not after all these years. He felt the sickness grow stronger as he thought about them and hurriedly got out of bed, becoming aware of his sore muscles. He noticed he was not at home but he decided that he was not going to think about that right now. There was a door standing ajar and behind it he saw a wash basin. He went into the bathroom, kneeled down in front of the toilet and leant forward but he couldn't vomit. For minutes he sat there, retching over the loo, shaking, while tears of shame, guilt and anger at himself stole their way out of his eyes.

This probably meant no chip for this month. And just yesterday he'd thought that he'd gotten pretty far with the programme...

After several more minutes he told himself that he was probably not going to vomit after all. He would have liked it in a way, it would have expressed his feelings quite adequately. In the dark he stumbled back into the room he had slept in, still not caring how he had gotten here in the first place, shuddering from nausea and weak from fatigue. As he laid down on the bed again he felt a cold sweat on his forehead. He wiped away the tears, hoping that no one he knew would ever see him like that. _Big boys don't cry_., he thought, _And they don't have wet dreams about shagging their own brothers_.

* * *

><p>When he woke up in the morning his head was still throbbing fiercely. He let out a little moan as he remembered his dream again. Luckily the sickly feeling in his throat had abated. Then he noticed what the mysterious heavy feeling on his chest and the warm substance against his left side meant. Someone was holding him. <em>Oh, I haven't slept with some random woman, have I?<em>, he thought, not wanting to open his eyes. He tried to remember last night but his memories were blurry at best. In the beginning of the evening he and Terry had talked to some people from work, that was all he knew for sure. He must have drunk a great deal to forget all the rest.

_It's not Terry, is it?_ He decided that he had to open his eyes to figure out who was lying next to him. He looked at the sleeping person, the daylight from the window increasing his headache. _Isn't he from Special Ops?_, he thought, just a little surprised to find a young man. He sometimes had one night stands with guys. He recalled that he had slept at a place unknown to him and was relieved that at least he'd been sensible enough to think of not taking the young man home. Don wasn't quite sure how his father would react.

_What's his name__ again?_, he thought. He cursed the pain in his head which made thinking so very hard. Special Ops were flown in from the headquarters for especially complicated or violent cases and he had only just met the guy sleeping next to him the day before yesterday. He hadn't really been working with him either since then. _How embarrassing_...

Then a flashback of him talking to the young man in a club flared through his mind. Had they really been talking about _physics_? How was it even possible they had landed in bed together? He dimly recollected that the other had explained at length some mathematical paradox about quantum theory to him. He'd understood only half of what the man had told him, probably even less. Then he remembered that while trying to look interested he'd been thinking _He's a bit like Charlie_.

Instantly the nauseating feeling inside him was back, he hurtled towards the bathroom again and this time he managed to vomit. He breathed hard for a few seconds before he flushed the toilet, trembling as he got up again. Was it possible to lose chips in the All Addicts Anonymous? He'd never heard of it before but sleeping with someone only because they reminded one of one's brother was a definite step backwards. Several steps, probably. All of them, more likely...

"Hey, are you alright? I've got some aspirin if you like.", a voice behind him said, obviously concerned.

He turned around to spot the man who had been sleeping next to him. He must have woken up while he had been in the bathroom. He even looked a little bit like Charlie. "Oh, I think it's just the alcohol. I was pretty drunk yesterday." He could barely stand looking at him and turned back towards the wash basin to thoroughly rinse his mouth and brush his teeth.

"Actually, that's not possible.", he heard the other say. "While you slept your body has processed all the alcohol you drank." Don looked into the mirror and saw his white face and the young man's worried brown eyes fixed on him from behind. "You see, the erythrocytes in your liver work as some kind of...", the male behind him began again but stopped as he saw how the serious eyes of the agent found his in the mirror. "You sure you're OK?", he said instead.

"An aspirin would be nice. I've got one hell of a headache.", he answered, looking down. _Charlie wouldn't know these things about how the body copes with alcohol. It has nothing to do with numbers_, he realised, and relief washed through him. Perhaps this wasn't as bad as it had seemed at first. He might even be allowed keep some of his chips...

Together they went back into the sleeping room and he watched the young Special Ops agent as he rummaged around in a suitcase in his boxers, probably looking for the medicine. _A suitcase_, Don thought, _perhaps we're in a hotel_. He noticed their clothes which were strewn all over the floor. _Must have been quite a night!_ The way his body was stiff and aching affirmed that last thought.

"There you go.", the other man had found the aspirin and handed it over.

"Thanks. Can you tell me where we are? I'm afraid I don't remember much about tonight.", he said and took a tablet into his mouth and swallowed it without water. _If I could just remember his name..._

The young man smiled at him. "Wellham's Hotel."

He nodded appreciatively, regretting it immediately when his head was throbbing harder at the movement. "Not bad. Now I know why you Special Ops people need such a high salary.", he tried to joke.

"Yeah, the expenses for us aren't exactly low.", the young man shrugged and smiled apologetically, "And to be accurate, we're BAU, not Special Operations."

"Oh, right." Don laid back onto the bed again to wait for the medicine to kick in. It was Saturday and he wasn't exactly busy. He intended to enjoy his remaining time at the luxury hotel and draped himself further into the still warm blanket, sighing a little as he did so. He watched the young man closer as he went into the bathroom to brush his teeth, too. From the back he didn't look like his little brother at all, he observed. He let his eyes linger on the other's bum and thought, _Perhaps I just slept with him because he's so damn attractive_ _and all this isn't what it seemed to be in the first place_.

He felt how the aspirin made him a little tired and how he was dozing off, finally at ease with the situation.

* * *

><p>A slight touch on his left shoulder arose him again. He blinked at the young man sitting on the bed next to him and smiled at him. "Spencer!" Now that his headache was gone his memory seemed to work better.<p>

"Just so you know, it's almost ten o'clock."

Don's gaze wandered to the bedside cabinet but there was no alarm clock because he wasn't at home. Then he looked at Spencer again who was just putting on a white shirt while he was still sitting. "Already getting ready to go?"

"I'm not exactly in a hurry but yeah, I should be leaving soon."

Don sat up, facing the other man. His hands reached out to touch the chest the still unbuttoned shirt was exposing, sliding them under the cloth and with one movement he pushed the fabric back, revealing slender shoulders. Spencer let the shirt glide down his arms and Don helped him out of it, then gently pulled the young man towards himself to place a kiss on his lips. "So you're 'not exactly in a hurry'...", he said in a low voice, sounding a little huskily, before he leaned in for a second kiss, his tongue slowly sliding along the other man's lower lip and making him moan into their kiss until it found its way into his mouth, meeting Spencer's tongue.

A small strand of saliva hung between them as they separated until it burst as Spencer drew back a little more. "Yeah, I guess I might find a little time for you before I go."

"Good.", Don said with a very serious expression on his face, his hands gliding to the other's hips, pulling him towards him even more and he laid back. Spencer straddled him and as he leant forwards their lips met again. He felt his chest being caressed, the other's finger rub his nipple. _Hell yes!_, he thought and got hard already, his tongue exploring that delicious mouth once more. He let his hands wander downwards, tightly gripping that bum he had admired before. Soon he was irritated by the fabric that separated him from the soft skin. He tore the waistband of the annoying boxers down with a fervour that seemed to surprise Spencer for he pulled back and looked at him with a raised eyebrow.

"You weren't quite that fierce last night.", he said with a coy smile.

"I wasn't?", he asked in wonderment. That didn't sound like him at all.

The other nodded. "Probably because you were plastered though. _Really_ plastered."

Don smirked, using one hand to pull the young agent's head closer to him for another kiss, the other to massage his finally freed bottom. As Spencer's lips wandered down his throat he didn't want to wait any longer. He stroked the other man's entrance and began to finger him, slowly first, but as he heard him moaning in pleasure he quickened his movements and soon added another finger. The other agent pushed back on his hand, his head hovering just above Don's, he had a blissed-out look and his half open eyes were misted.

He reached for the lube he had spotted on the bedside cabinet before and whispered, "Tell me you want me!"

Spencer moaned and barely managed to croak out, "Yes, yes, I want you!"

He pulled his fingers out to apply the lube and then he put his hands on the other man's hips to guide him. Little by little he lifted his own hips and slid his cock into Spencer, his slow advance obviously antagonising the other who fiercely pushed back so that Don was pressed on the bed again. Then Spencer began to rock his hips and, _Damn, he's good!_, he thought. He gently stroked the other's chest and let him define the pace.

"You like that?", Don gasped, not to find out whether Spencer was all right but just because he wanted to hear him say that yes, he _did_ like it.

The younger man put his hands on either side of his head, now being able to move even quicker on Don, and moaned, "God, yes!"

"Say it!", Don demanded, watching that handsome face hover above him.

"I... I like riding you!", Spencer replied panting.

He bit his lip and watched the young man, the faintest trace of a leer in his eyes. His hands wandered back to the other's hips, caressing the deliciously soft skin while he was eyeing Spencer's arousal brush against his stomach with interest.

Suddenly the young man on him leant forward to whisper something into his ear. "Did you know", his hoarse voice mumbled, and he sounded just like Charlie, "that the Tschirnhausen transformation is a type of mapping on polynominals-"

"Fuck!", Don called out just before he came and he meant it, because nothing had ever made him come so fast than this math talk he didn't even understand. But the ecstasy he felt flashing through his body erased all guilty feelings he might have had. He arched his back, pressing hardly, desperately into Spencer as the young man whispered something about "an irreducible polynomial which takes a root to some rational function-" into his ear. "Oh fuck, yes!", he shouted again, sensing the blissful rapture slowly fade away already, but as the man riding him went on talking about some "quotient ring", "alpha" and "beta" and "primitive elements" he felt that he was coming _again_. He pulled the other man with force down on him while he was pushing upwards frantically, clinging very closely to him, every movement sending new spikes of pleasure through him. "Yes!", he yelled and hit the peak another time, hearing that, finally, Spencer's ramblings were also getting incoherent, the last thing he heard was some unintelligible muttering about a "Galois theory", before he was reduced to a litany of "Yes"'s, too.

As Spencer had at last spilled his semen onto Don's stomach he gently pulled out and let go of the young agent. His mind was still swimming in the aftermath of the pleasure he had just experienced but now the realisation of what had happened seeped into his mind as well. Math talk turned him on. No, he had to correct himself, thinking of his little brother had done it for him. _Not good_, he thought, _That's definitely no new chip for me this month.._. Somehow, the nausea didn't appear this time, though. He was exhausted and felt his sore muscles ache but felt someway elated. He wondered whether this was good or bad. The nausea had always been some sort of boundary that had prevented him from deliberately touching himself while he was thinking of Charlie. Would it be gone forever or just now that he had experienced the best sex he'd ever had?

Don turned to the young man who laid next to him, breathing hard, and asked, "How did you know that I like that?"

"You asked me to talk about equations the moment we were in this hotel room."

He shook his head in disbelief. _Fuck_, he thought, _I must never_ _get that drunk again!_

"Oh shoot, I really should be off now!", Spencer said and got up. Don watched him as he gathered up his clothes and got dressed hurriedly. As he was fastening his shoes his mobile phone rang.

"Reid? – Oh, yeah, I'm on my way, sorry, I overslept. – I'll be there in no time, really! – Yeah, see you at the garage!" As he put the phone back into his pocket he got up and took his already packed suitcase. He turned to Don, "Oh, take all the time you need, I'll give them the key but I think you can still get out without one."

"Um. Spencer?"

"Yeah?"

"Can I have your card?"

The young man raised his eyebrow. "Oh... sure." He fumbled in his inner pocket of his jacket and put his business card on the bedside cabinet. "See you!", he said with a little smile and left the room.

As the door clicked shut Don reached out for the card and read the address. "Fuck! Why does it have to be fucking Virginia?"


End file.
